This invention relates to adaptive beamformers and more particularly to adaptive beamformers for use in sonar and radar receivers which are capable of discriminating against interfering sources while providing the desired target signal.
Arrays of sonar transducers or radar receivers have a maximum response axis or antenna beam direction which can be steered by applying linear phase weighting across a linear array. The beam can be shaped by amplitude and phase weighting of the outputs of the array elements. Most arrays are built with fixed weights designed to produce a pattern that is a compromise between resolution, gain and low sidelobes. Adaptive systems that can sense and respond to a time-varying environment have been applied to arrays in order to reduce the susceptibility of the receiver attached to the array to jamming or interference. The degradation in the signal-to-noise ratio performance of a receiving system to the desired signal is caused by undesired noise which intrudes via the arra sidelobe and mainlobe. The noise may consist of a deliberate interfering signal and natural noise sources. The degradation and signal-to-noise is often further aggravated by multi-path and a changing interference environment. Adaptive array techniques have been utilized as solutions to removing interference by the flexible capabilities for automatic null steering in the spatial domain.
Many adaptive array systems have been proposed with a typical adaptive array performing spatial filtering by sensing automatically the direction of a source of interference and forming a retrodirective receive beam in that direction to subtract from its normal (unadapted) beam which is responsive to the source of interference on one of its sidelobes. The retrodirective beam denotes a receive beam automatically formed in the direction of a single source of interference. The adaptive array forms a retrodirective receive beam by cross-correlating the received element signals with a received reference signal. The reference signal may con ist of the output of a separate antenna or the output of the array. In either case, the basic principle is that of the cross-correlation interferometer.
These sophisticated adaptive beamformers, which can effectively remove any number of interfering sources from the beamformer outputs, are relatively complex and expensive.